Contemporary vehicles including passenger cars, light duty trucks, and medium duty vehicles use evaporative emissions systems to prevent unnecessary emission of hydrocarbon (HC) vapors into the atmosphere. These emissions are primarily composed of gasoline vapors leaking from a vehicle's fuel tank to the air. In a typical system, the fuel tank is periodically vented into a canister filled with charcoal that filters the HC vapors and releases the filtered air to the atmosphere. The charcoal traps the hydrocarbon molecules from the polluting vapors, preventing them from leaking to the atmosphere.
Near term emissions regulation requires the monitoring of the vehicle's evaporative emission system to ensure integrity of operation. This requirement specifies, among other things, checking of the absence of leaks in the system. More specifically, the California Air Resources Board, CARB, specifies in their proposed On Board Diagnostic II, (OBD II) requirement to check the evaporative emissions system for leaks. This requires detecting system leaks equivalent to an orifice larger than 0.020 inches in diameter for vehicles produced in model year 2000.
Most prior art schemes for leak detection in an evaporative emissions system utilize a measurement of pressure internal to the fuel tank in order to detect a leak. These measurements are typically made over a period of time on the order of two minutes (one-hundred and twenty seconds) or more.
The prior art leak detection schemes, however, are particularly susceptible to large fluctuations in the fuel tank pressure caused by road load disturbances or violent driver inputs to the vehicle itself. For example, pot holes and railroad tracks can apply powerful impulses to a vehicle's suspension system. Furthermore, cobblestone streets or even rough pavement can excite the vehicle suspension for an extended period of time. These types of road load disturbances can cause a pressure measurement based leak detection system to make a faulty diagnosis by artificially raising or lowering the fuel tank pressure. For example, rocking a warm car on a warm day with high RVP fuel will cause tank pressure to rise faster than without the added suspension excitement. For a car with a cold fuel tank, sloshing fuel can actually be chilled by the cold tank walls, lowering vapor generation and keeping tank pressure artificially low. Misdiagnosis is a serious problem, as failing to properly diagnose a fault can result in a CARB mandated recall, while diagnosing a compliant system as having a leak will drive up OEM warranty costs.
What is needed is an improved method of detecting a leak in an evaporative emissions system for a vehicle that is more accurate than prior art schemes due to the system's ability to prevent road load disturbances or violent vehicle maneuvering from causing erroneous detection or non-detection of leaks.